Use of an ink jet recording system has been promoted in various fields due to low noise in recording, easy coloring and capability of rapid recording. However, papers generally used in printing, such as wood free paper, are poor in ink absorption, drying and image quality such as resolution. Thus papers for an exclusive use with improved properties for the above problems have been proposed. For example, a recording paper coated with various inorganic pigments including amorphous silica to enhance coloring of ink and reproducibility, has been disclosed (for example, JP-A-55-51583 and JP-A-56-148585).
However, since further performance improvement in the recording medium side has been required along with progress in performance of ink jet printers, satisfactory performance can not be obtained at present by the above-described technology alone. In particular, an increase in ink discharge per unit area of recording medium has now been required to obtain an image quality as high as that of silver halide photograph. Thus, this demand brings about problems such as poor ink absorption and the occurrence of bleeding in the recording medium. This further requires transparency of an ink absorption layer and water resistance of the printed part after printing to realize high image quality, surface gloss and expression of color density equivalent to those of silver halide photograph.
With recent progress in technical development of an ink jet recording medium to obtain a high image quality equivalent to silver halide photograph, a recording medium is now commercially available, which gives an image quality similar to a photograph, by providing an ink absorption layer having micro-pores formed by inorganic fine particles on a highly smooth substrate such as a paper coated with polyolefin and a polymer film. However, in order to secure a sufficient ink absorption capacity on a substrate without ink absorption ability by using only said ink absorption layer, a very thick ink absorption layer of at least 35 μm must be formed. To obtain such a thick ink absorption layer, an uniform application of coating liquid in a thickness as thick as several times of the dry thickness is required. But it is difficult and troublesome to manufacture said recording medium efficiently while keeping a uniform film thickness even during the drying process. Furthermore, since it is recently required to use water as the solvent for the coating liquid in view of global environmental protection, said problem has become more critical because water requires generally a longer drying time than do organic solvents. Thus an effective solution is now needed.
As one of the solutions, a method for manufacturing a recording medium using a coating liquid containing polyvinyl alcohol and boric acid and/or borax together with inorganic fine particles has been disclosed (JP-A-2000-218927). Said coating liquid keeps a state of aqueous solution with a low viscosity at a relatively high temperature (about 40° C. or higher), whereas it shows thickening (gelling) at a low temperature in the region of around 15° C. Said coating liquid also shows similar thickening with a temperature change when it contains gelatin together with inorganic fine particles. A thick coating layer can be retained uniformly by utilizing said property. However, use of said water soluble polymers such as polyvinyl alcohol and gelatin has a problem of clogging micro-pores formed by inorganic fine particles, and in particular, use of gelatin makes this problem more critical. Use of polyvinyl alcohol and boric acid and/or borax also has a risk of water pollution caused by boron, in view of recent environmental regulations relating to water pollution becoming more strict regarding boron. Increasing the solids content of coating-liquid-containing-water soluble-polymers such as polyvinyl alcohol and gelatin, to reduce the amount of water to be evaporated; and reducing the drying time to save energy required for drying, causes problems such as difficulties in handling and uniform application of coating liquid on a substrate due to an abrupt increase in the viscosity of the coating liquid caused by the increase in concentration.
Furthermore, a binder composition using a polymer compound showing reversible change between hydrophilicity and lipophilicity at a specific temperature (temperature-sensitive point) (JP-A-2001-253996) and a method for manufacturing a recording medium utilizing said polymer compound (JP-A-2001-180105) and the like have been disclosed. All utilize the thickening effect by heating a low viscosity binder composition or a coating liquid prepared at a temperature range (not higher than the temperature-sensitive point) at which said polymer compound is hydrophilic, at a temperature not lower than the temperature-sensitive point. A coating layer obtained by using said binder composition or said coating liquid is poor in transparency, surface smoothness and surface gloss, and thus can not provide an ink jet recording medium giving a high image quality equivalent to silver halide photograph.
Thus, developments are needed for an ink jet recording medium sufficiently acceptable in view of image quality, global environmental preservation and energy saving, a coating liquid used for manufacturing said recording medium, an efficient method for manufacturing said recording medium, a polymer emulsion useful for manufacturing said recording medium and a method for manufacturing said polymer emulsion.
Furthermore, even in such recording medium as used in thermal recording printing by using coloring of an ink composition by heating, gravure printing, offset printing and other various printing systems as well as recording with recording tools such as a pen, developments are needed for a recording medium sufficiently acceptable in view of surface gloss, image quality, global environmental preservation and energy saving, a coating liquid used in manufacturing said coating medium, an efficient method for manufacturing said recording medium, a polymer emulsion useful for manufacturing said recording medium and a method for manufacturing said polymer emulsion.